FinFET SRAM Having Discontinuous PMOS Fin Lines

ABSTRACT

An IC chip includes a logic circuit cells array and a static random access memory (SRAM) cells array. The logic circuit cells array includes a plurality of logic circuit cells abutted to one another in a first direction. The logic circuit cells array includes one or more continuous first fin lines that each extends across at least three of the abutted logic circuit cells in the first direction. The static random access memory (SRAM) cells array includes a plurality of SRAM cells abutted to one another in the first direction. The SRAM cells array includes discontinuous second fin lines.

PRIORITY DATA

This application is a continuation of U.S. application Ser. No.16/101,728, filed Aug. 13, 2018, which is a continuation of U.S.application Ser. No. 15/492,777, filed Apr. 20, 2017, now U.S. patentSer. No. 10/056,390, issued Aug. 21, 2018, which are herein incorporatedby reference in their entirety.

BACKGROUND

In deep sub-micron integrated circuit technology, an embedded staticrandom access memory (SRAM) device has become a popular storage unit ofhigh speed communication, image processing and system-on-chip (SOC)products. The amount of embedded SRAM in microprocessors and SOCsincreases to meet the performance requirement in each new technologygeneration. As silicon technology continues to scale from one generationto the next, the impact of intrinsic threshold voltage (Vt) variationsin minimum geometry size bulk planar transistors reduces thecomplimentary metal-oxide-semiconductor (CMOS) SRAM cell static noisemargin (SNM). This reduction in SNM caused by increasingly smallertransistor geometries is undesirable. SNM is further reduced when Vcc isscaled to a lower voltage.

To solve SRAM issues and to improve cell shrink capability, fin fieldeffect transistor (FinFET) devices are often considered for someapplications. The FinFET provides both speed and device stability. TheFinFET has a channel (referred to as a fin channel) associated with atop surface and opposite sidewalls. Benefits can be derived from theadditional sidewall device width (I_(on) performance) as well as bettershort channel control (sub-threshold leakage). Therefore, FinFETs areexpected to have advantages in terms of gate length scaling andintrinsic V_(t) fluctuation. However, existing FinFET SRAM devices stillhave shortcomings, for example shortcomings related to cell writemargins or chip speeds.

Therefore, although existing FinFET SRAM devices have been generallyadequate for their intended purposes, they have not been entirelysatisfactory in every aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion. It is also emphasized that thedrawings appended illustrate only typical embodiments of this inventionand are therefore not to be considered limiting in scope, for theinvention may apply equally well to other embodiments.

FIG. 1A is a perspective view of an example FinFET device.

FIG. 1B illustrates a diagrammatic cross-sectional side view of FinFETtransistors in a CMOS configuration.

FIG. 2 illustrates a top view of a standard (STD) cells array accordingto embodiments of the present disclosure.

FIG. 3 illustrates a top view of an SRAM cells array according toembodiments of the present disclosure.

FIG. 4 illustrates a top view of a standard (STD) cells array accordingto embodiments of the present disclosure.

FIG. 5 illustrates a top view of an SRAM cells array according toembodiments of the present disclosure.

FIG. 6A illustrates circuit schematics of various logic gates accordingto some embodiments of the present disclosure.

FIG. 6B illustrates the top view of the layout corresponding to thelogic gates shown in FIG. 6A according to some embodiments of thepresent disclosure.

FIG. 6C illustrates a diagrammatic fragmentary cross-sectional view ofthe corresponding cells shown in FIG. 6B according to some embodimentsof the present disclosure.

FIG. 7A illustrates a circuit schematic for a single-port SRAM cellaccording to embodiments of the present disclosure.

FIG. 7B illustrates the layout in a top view of the single-port SRAMcell shown in FIG. 7A according to embodiments of the presentdisclosure.

FIG. 8A illustrates a cross-sectional side view of two abutting SRAMcells according to embodiments of the present disclosure.

FIG. 8B illustrates the layout of the two abutting SRAM cells of FIG. 8Ain a top view according to embodiments of the present disclosure.

FIG. 9A is a diagrammatic fragmentary cross-sectional side view of aportion of a CMOSFET device in a standard cell according to embodimentsof the present disclosure.

FIG. 9B is a diagrammatic fragmentary cross-sectional side view of aportion of a CMOSFET device in an SRAM cell according to embodiments ofthe present disclosure.

FIG. 10 is a diagrammatic fragmentary cross-sectional side view of aportion of an interconnect structure according to embodiments of thepresent disclosure.

FIG. 11 is a flowchart illustrating a method according to an embodimentof the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is directed to, but not otherwise limited to, afin-like field-effect transistor (FinFET) device. The FinFET device, forexample, may be a complementary metal-oxide-semiconductor (CMOS) deviceincluding a P-type metal-oxide-semiconductor (PMOS) FinFET device and anN-type metal-oxide-semiconductor (NMOS) FinFET device. The followingdisclosure will continue with one or more FinFET examples to illustratevarious embodiments of the present disclosure. It is understood,however, that the application should not be limited to a particular typeof device, except as specifically claimed.

The use of FinFET devices has been gaining popularity in thesemiconductor industry. Referring to FIG. 1A, a perspective view of anexample FinFET device 50 is illustrated. The FinFET device 50 is anon-planar multi-gate transistor that is built over a substrate (such asa bulk substrate). A thin silicon-containing “fin-like” structure(hereinafter referred to as a “fin”) forms the body of the FinFET device50. The fin extends along an X-direction shown in FIG. 1A. The fin has afin width W_(fin) measured along a Y-direction that is orthogonal to theX-direction. A gate 60 of the FinFET device 50 wraps around this fin,for example around the top surface and the opposing sidewall surfaces ofthe fin. Thus, a portion of the gate 60 is located over the fin in aZ-direction that is orthogonal to both the X-direction and theY-direction.

L_(G) denotes a length (or width, depending on the perspective) of thegate 60 measured in the X-direction. The gate 60 may include a gateelectrode component 60A and a gate dielectric component 60B. The gatedielectric 60B has a thickness t_(ox) measured in the Y-direction. Aportion of the gate 60 is located over a dielectric isolation structuresuch as shallow trench isolation (STI). A source 70 and a drain 80 ofthe FinFET device 50 are formed in extensions of the fin on oppositesides of the gate 60. A portion of the fin being wrapped around by thegate 60 serves as a channel of the FinFET device 50. The effectivechannel length of the FinFET device 50 is determined by the dimensionsof the fin.

FIG. 1B illustrates a diagrammatic cross-sectional side view of FinFETtransistors in a CMOS configuration. The CMOS FinFET includes asubstrate, for example a silicon substrate. An N-type well and a P-typewell are formed in the substrate. A dielectric isolation structure suchas a shallow trench isolation (STI) is formed over the N-type well andthe P-type well. A P-type FinFET 90 is formed over the N-type well, andan N-type FinFET 91 is formed over the P-type well. The P-type FinFET 90includes fins 95 that protrude upwardly out of the STI, and the N-typeFinFET 91 includes fins 96 that protrude upwardly out of the STI. Thefins 95 include the channel regions of the P-type FinFET 90, and thefins 96 include the channel regions of the N-type FinFET 91. In someembodiments, the fins 95 are comprised of silicon germanium, and thefins 96 are comprised of silicon. A gate dielectric is formed over thefins 95-96 and over the STI, and a gate electrode is formed over thegate dielectric. In some embodiments, the gate dielectric includes ahigh-k dielectric material, and the gate electrode includes a metal gateelectrode, such as aluminum and/or other refractory metals. In someother embodiments, the gate dielectric may include SiON, and the gateelectrode may include polysilicon. A gate contact is formed on the gateelectrode to provide electrical connectivity to the gate.

FinFET devices offer several advantages over traditional Metal-OxideSemiconductor Field Effect Transistor (MOSFET) devices (also referred toas planar transistor devices). These advantages may include better chiparea efficiency, improved carrier mobility, and fabrication processingthat is compatible with the fabrication processing of planar devices.Thus, it may be desirable to design an integrated circuit (IC) chipusing FinFET devices for a portion of, or the entire IC chip.

However, traditional FinFET fabrication methods may still haveshortcomings, such as lack of optimization for embedded SRAMmanufacturing. For example, traditional FinFET fabrication may faceconcerns related to SRAM cell write margin and logic circuit speeds. Thepresent disclosure describe FinFET logic circuit and SRAM cells thathave improved SRAM cell write margin without reducing the logic circuitspeeds, as discussed in more detail below.

FIG. 2 illustrates a top view of a standard (STD) cells array 100according to embodiments of the present disclosure. The standard cellsarray 100 may include logic circuits or logic devices, and as such it isalso referred to as a logic cells array or a logic circuit array. Invarious embodiments, the logic circuits or devices may includecomponents such as inverters, NAND gates, NOR gates, flip-flops, orcombinations thereof.

As illustrated in FIG. 2, the standard cells array 100 includes N-typeFinFET transistors with a P-type well, as well as P-type FinFETtransistors with an N-type well. The standard cells array 100 alsoincludes a plurality of elongated fin lines, for example fin lines110-111 as parts of the P-type FinFET transistors, as well as fin lines120-121 as parts of the N-type FinFET transistors. The P-type FinFET finlines 110-111 are located over the N-type wells, whereas the N-typeFinFET fin lines 120-121 are located over the P-type wells.

As an example, the standard cells array 100 shown herein includes 10standard cells 131 through 140, where the cells 131 through 135 arearranged into a first column, and the cells 136 through 140 are arrangedinto a second column adjacent to the first column. Of course, FIG. 2merely illustrates an example of the standard cells array 100, and otherembodiments may have different numbers of cells and/or may be arrangeddifferently.

As shown in FIG. 2, the fin lines 110-111 and 120-121 each extendthrough a respective column of the standard cells (e.g., fin lines 110and 120 extending through the standard cells 1-5, and fin lines 111 and121 extending through the standard cells 6-10) in the X-direction(X-direction of FIG. 1A). Thus, the fin lines 110-111 and 120-121 mayeach be considered “continuous.”

As discussed above with reference to FIG. 1A, the fin lines 110-111 and120-121 each include a channel region as well as source/drain regionslocated next to (e.g., on opposite sides of) the channel region. TheFinFET transistors of the STD cells array 100 each include a respectivegate electrode that wraps around a respective one of the fin lines110-111 or 120-121 in the manner described above with reference to FIG.1A. In the present embodiments, the P-type FinFET (PMOSFET) fin lines110-111 are comprised of a silicon germanium (SiGe) material (forenhancing the strain effect), but the N-type FinFET (NMOSFET) fin lines120-121 are comprised of a non-germanium-containing semiconductormaterial, for example silicon (Si). Therefore, in some embodiments, thePMOSFET has a SiGe channel, but the NMOSFET has a Si channel. In someembodiments, a channel fin width of the NMOSFET is narrower than achannel fin width of the PMOSFET. In some embodiments, the source/drainregions of the NMOSFET includes an epi-material selected from the groupconsisting of: SiP, SiC, SiPC, SiAs, Si, or combinations thereof. Insome embodiments, the PMOSFET's source/drain region has a wider widththan the channel region.

In some embodiments, for the PMOSFET, the germanium atomic concentrationin the SiGe channel region is less than the germanium atomicconcentration in the source/drain region. For example, the germaniumatomic concentration in the SiGe channel region may be in a rangebetween about 10% and about 40%, and the germanium atomic concentrationin the source/drain region may be in a range between about 30% and about75% in some embodiments.

In some embodiments, for the PMOSFET, the SiGe channel fin width issmaller than the SiGe channel sidewall depth. For example, the SiGechannel fin width for the PMOSFET may be in a range between about 3nanometers (nm) and about 10 nm, and the SiGe channel sidewall depth(labeled in FIG. 1A as channel sidewall depth 85) may be in a rangebetween about 30 nm and about 90 nm in some embodiments.

As discussed above, each of the fin lines 110-111 and 120-121 of thestandard cells array 100 is continuous. For example, the fin lines110-111 and 120-121 each extend across at least three abutted cells(e.g., cells abutted in the X-direction). In the embodiment shown inFIG. 2, the fin lines 110 and 120 each extend across five abuttedstandard cells 1-5, and the fin lines 111 and 121 each extend acrossfive other abutted standard cells 6-10.

Referring now to FIG. 3, a top view of an SRAM cells array 200 isillustrated according to embodiments of the present disclosure. The SRAMcells array 200 includes SRAM cells, for example SRAM cells 210-217. Inthe illustrated embodiment, the SRAM cells 210-213 are arranged into afirst column extending in the X-direction (of FIG. 1A), and the SRAMcells 214-217 are arranged into a second column also extending in theX-direction, where the first column is disposed adjacent to the secondcolumn in the Y-direction (of FIG. 1A). Each of the SRAM cells 210-217comprises two pull-up (PU) transistors, two pass-gate (PG) transistors,and two pull-down (PD) transistors, which may be implemented as FinFETs.

The SRAM cells array 200 includes a plurality of elongated fin lines,for example the fin lines 220-224 and 230-234 as parts of the P-typeFinFET transistors in the pull-up (PU) portion of the SRAM cells array200, as well as fin lines 240-243 as parts of the N-type FinFETtransistors in the pass-gate (PG) and pull-down (PD) portions of theSRAM cells array 200. The P-type fin FinFET lines 220-224 and 230-234are located over the N-type wells, whereas the N-type FinFET fin lines240-243 are located over the P-type wells.

The fin lines 220-224, 230-234, and 240-243 each extend into one or moreof respective SRAM cells in the X-direction. For example, the N-typeFinFET fin lines 240-241 each extend continuously across the SRAM cells210-213, and the N-type FinFET fin lines 242-243 each extendcontinuously across the SRAM cells 214-217. In comparison, the P-typeFinFET fin lines 220-224 and 230-234 are “discontinuous” or “disjointedwith one another.” For example, the P-type FinFET fin line 220 extendspartially into the SRAM cell 210, the fin line 221 extends partiallyinto the SRAM cells 210-211, the fin line 222 extends partially into theSRAM cells 211-212, the fin line 223 extends partially into the SRAMcells 212-213, and the fin line 224 extends partially into the SRAM cell213. The fin line 221 overlaps with the fin lines 220 and 222 in theX-direction but is spaced apart from the fin lines 220 and 222 in theY-direction. Likewise, the fin line 223 overlaps with the fin lines 222and 224 in the X-direction but is spaced apart from the fin lines 222and 224 in the Y-direction.

In the adjacent column of cells 214-217, the P-type FinFET fin line 230extends partially into the SRAM cell 214, the fin line 231 extendspartially into the SRAM cells 214-215, the fin line 232 extendspartially into the SRAM cells 215-216, the fin line 233 extendspartially into the SRAM cells 216-217, and the fin line 234 extendspartially into the SRAM cell 217. The fin line 231 overlaps with the finlines 230 and 232 in the X-direction but is spaced apart from the finlines 230 and 232 in the Y-direction. Likewise, the fin line 233overlaps with the fin lines 232 and 234 in the X-direction but is spacedapart from the fin lines 232 and 234 in the Y-direction.

As discussed above with reference to FIG. 1A, the fin lines 220-224,230-234 and 240-243 each include a channel region as well assource/drain regions located next to (e.g., on opposite sides of) thechannel region. The FinFET transistors each include a gate electrodethat wraps around a respective one of the fin lines 220-224, 230-234,and 240-243 in the manner described above with reference to FIG. 1A. Inthe present embodiments, the P-type FinFET fin lines 220-224 and 230-234are comprised of a silicon germanium (SiGe) material (for strain effectenhancement), but the N-type FinFET fin lines 240-243 are comprised of anon-germanium-containing material, for example Si.

It can be seen that, whereas both the fin lines 110-111 for the P-typeFinFETs and the fine lines 120-121 for the N-type FinFETs in thestandard cells array 100 shown in FIG. 2 are each continuous, and thefin lines 240-243 for the N-type FinFETs in the SRAM cells array 200shown in FIG. 3 are continuous, the fin lines 220-224 and 230-234 forthe P-type FinFETs in the SRAM cells array 200 are “discontinuous.” Forexample, the P-type FinFET fin lines 220-224 could have been implementedas a single continuous fin line (e.g., similar to the N-type FinFET finline 240) spanning across the SRAM cells 210-213, but according to thevarious aspects of the present disclosure, that hypothetical single finline is broken up into five discrete and separate fin lines 220, 221,222, 223, and 224. The fin lines 220 and 222 are separated by a gap 250that spans across the boundary between the SRAM cells 210-211 (in theX-direction), and the fin lines 222 and 224 are separated by a gap 251that spans across the boundary between the SRAM cells 212-213 (in theX-direction). The fin lines 221 and 223 are separated by a gap 252 thatspans across the boundary between the SRAM cells 211-212 (in theX-direction). Due at least in part to these gaps 250-252, it may be saidthat the P-type FinFETs in the SRAM cells 200 have discontinuous orbroken-up fin lines.

The fin lines 230-234 in the SRAM cells 214-217 are arranged (i.e.,broken up into discontinuous fin lines) in a similar manner as the finlines 230-234. Thus, although each of the fin lines 220-224 and 230-234extend partially across two adjacent SRAM cells, it may be said that theSRAM cells array 200 has an overall “discontinuous” fin line shape forits P-type FinFETs, which is not the case for the standard cells array100 or for the N-type FinFETs of the SRAM cells array 200. In someembodiments, the end of each “discontinuous” fin line is located under agate electrode of another CMOSFET. In some embodiments, thediscontinuous or disjointed fin lines 220-224 and 230-234 each extendsinto no more than two adjacently disposed SRAM cells.

The reason that the fin lines for the standard cells array 100 arecontinuous but the fin lines (for P-type FinFET) for the SRAM cellsarray 200 are discontinuous is due to the I_(on) (on current) concerns.If the P-type FinFET devices for the SRAM cells have continuous finlines, the I_(on) current would be too high, which is not good for SRAMwrite margins. According to the present disclosure, the P-type FinFETfin lines for the SRAM cells array 200 are “broken up” or configured ina “discontinuous” manner. This destroys or reduces the strain effect(for the SiGe-strained channels). Consequently, the I_(on) current isreduced for the P-type FinFET fin lines of the SRAM cells array 200,thereby relaxing the SRAM writing margin concerns. Meanwhile, thecontinuous fin lines are good for logic circuit speeds. The continuousfin lines also solve problems related to line-end shrinkage controlproblems PMOSFET layout dependent effects for logic circuits. As such,the logic cells (or STD cells) are configured to have continuous finlines.

FIGS. 4-5 illustrate another embodiment of the STD cells array 100 andthe SRAM cells array 200, respectively. The embodiment of the STD cellsarray 100 and the SRAM cells array 200 are similar to the embodimentshown in FIGS. 2-3, and thus the similar elements appearing in bothembodiments are labeled the same herein. However, the embodiment of theSTD cells array 100 shown in FIG. 4 does not have the N-type FinFET finlines 120-121, and the embodiment of the SRAM cells array 200 shown inFIG. 5 does not have the N-type FinFET fin lines 240-243. Nevertheless,the embodiment of the SRAM cells array 200 shown in FIG. 5 still hasdiscontinuous or broken fin lines for its P-type FinFETs for the samereasons (e.g., I_(on) current) as discussed above.

FIGS. 6A, 6B, 6C illustrate one or more standard cells according to someembodiments of the present disclosure. In more detail, FIG. 6Aillustrates the circuit schematics of some common logic gates builtusing CMOS FinFETs, FIG. 6B illustrates the top view layoutcorresponding to these logic gates shown in FIGS. 6A, and 6C illustratesa diagrammatic fragmentary cross-sectional side view of thecorresponding cells shown in FIG. 6B. It is understood that the top viewlayout shown in FIG. 6B may correspond to one or more of the STD cells(or portions thereof) shown in FIG. 2 or 4.

As examples, the logic gates shown in FIG. 6A includes an inverter gate,a NAND gate, and a NOR gate. The inverter gate, the NAND gate, and theNOR gate each include one or more N-type MOSFETs (NMOSFET) and one ormore P-type MOSFETs (PMOSFETs). The particular type of logic gate isdetermined by coupling the gate, source, and drain of the NMOSFETs andPMOSFETs in a specific configuration as shown in FIGS. 6A-6B. The inputterminal and output terminal of each logic gate is also labeled in FIG.6A as such.

The top view layout of FIG. 6B illustrates PMOSFETs with an N-type wellregion and NMOSFETs with a P-type well region. A plurality of elongatedfin lines 310-311 and 320-321 extend in an elongated manner in theX-direction. The fin lines 310-311 are parts of the PMOSFET, and the finlines 320-321 as parts of the NMOSFET. The PMOSFET fin lines 310-311 arelocated over the N-type well region, whereas the NMOSFET fin lines320-321 are located over the P-type well region.

As discussed above with reference to FIG. 1A, the fin lines 310-311 and320-321 each include a channel region as well as source/drain regionslocated next to (e.g., on opposite sides of) the channel region. In thepresent embodiments, the PMOSFET fin lines 310-311 are comprised of asilicon germanium (SiGe) material (for strain effect enhancement), butthe NMOSFET fin lines 320-321 are comprised of anon-germanium-containing semiconductor material, for example Si. The finlines 310-311 and 320-321 are each continuous, for example they eachextend across three or more abutted cells (abutted in the X-direction).

In each of the circuit cells (e.g., the inverter, NAND, or NOR), one ormore CMOS gates 350 extend into both the N-type well region and theP-type well region in the Y-direction. The portion of the gate 350located over the N-type well region forms the gate of the PMOSFET, andthe portion of the gate 350 located over the P-type well region formsthe gate of the NMOSFET. Each of the gates 350 wraps around the finlines 310-311 and 320-321 in the manner described above with referenceto FIG. 1A. For example, the gates 350 in the PMOSFET wrap around thefin lines 310-311, and the gates 350 in the NMOSFET wrap around the finlines 320-321. The source/drain contacts (providing electricalconnectivity to the source/drains of the FinFETs) are also illustratedin the top view layout of FIG. 6B, some examples of which are labeledherein as source contacts 370 and drain contacts 380. It is understoodthat silicide layers may be formed on the source/drain regions, and thesource/drain contacts may be formed on the silicide layers.

According to the various aspects of the present disclosure, a pluralityof isolation transistors is implemented between adjacent cells toprovide electrical isolation between the adjacent circuit cells. In moredetail, PMOSFET isolation transistors include gates 400, and the NMOSFETisolation transistors include gates 410. The gates 400-410 are eachlocated on a border between two adjacent circuit cells, for example onthe border between the inverter cell and the NAND cell, on the borderbetween the NAND cell and the NOR cell, etc. The gates 400 of thePMOSFET isolation transistors are each tied to a voltage source Vdd, andthe gates 410 of the NMOSFET isolation transistors are each tied to avoltage source Vss.

For the PMOSFET isolation transistors, their gates 400 wrap around thefin lines 310-311 having the SiGe channels. The source region of thePMOSFET isolation transistor is common with the P-type source/drainregion of one of the PMOSFET transistors from the standard cells, andthe drain region of the PMOSFET isolation transistor is common with theP-type source/drain region of another one of the PMOSFET transistorsfrom the standard cells. Likewise, for the NMOSFET isolationtransistors, their gates 410 wrap around the fin lines 320-321 havingthe Si channels. The source region of the NMOSFET isolation transistoris common with the N-type source/drain region of one of the NMOSFETtransistors from the standard cells, and the drain region of the NMOSFETisolation transistor is common with the N-type source/drain region ofanother one of the NMOSFET transistors from the standard cells.

Due at least in part to their locations (e.g., the gates 410 beinglocated on the circuit cell borders) and their electrical configuration(e.g., the gates 410 being electrically tied to Vdd), the PMOSFETisolation transistors provide electrical isolation between the adjacentcircuit cells for the PMOSFET, for example between the inverter cell andthe NAND cell, or between the NAND cell and the NOR cell. Similarly, theNMOSFET isolation transistors provide electrical isolation between theadjacent circuit cells for the NMOSFET, for example between the invertercell and the NAND cell, or between the NAND cell and the NOR cell.

The cross-sectional side view of FIG. 6C is obtained by cutting along acutline 450 in the N-type well region of the top view of the standardcells layout of FIG. 6B. As shown in FIG. 6C, the standard cell has anN-type well formed in a silicon substrate. The continuous fin line 310is formed over the N-type well. A plurality of source and drain regions(including common node) is formed in the fin line 310, and plurality ofgates is formed over the fin line 310. Some of these gates are the gates400 of the isolation transistors discussed above. A plurality ofcontacts (CO) is formed over the source and drain regions to provideelectrical connectivity thereto.

FIG. 7A illustrates a circuit schematic for a single-port SRAM cell 500,and FIG. 7B illustrates the corresponding layout in a top view of thesingle-port SRAM cell 500 according to embodiments of the presentdisclosure. The single-port SRAM cell 500 includes pull-up transistorsPU1, PU2; pull-down transistors PD1, PD2; and pass-gate transistors PG1,PG2. As show in the circuit diagram, transistors PU1 and PU2 are p-typetransistors, such as the p-type FinFETs discussed above, and transistorsPG1, PG2, PD1, and PD2 are n-type FinFETs discussed above.

The drains of pull-up transistor PU1 and pull-down transistor PD1 arecoupled together, and the drains of pull-up transistor PU2 and pull-downtransistor PD2 are coupled together. Transistors PU1 and PD1 arecross-coupled with transistors PU2 and PD2 to form a first data latch.The gates of transistors PU2 and PD2 are coupled together and to thedrains of transistors PU1 and PD1 to form a first storage node SN1, andthe gates of transistors PU1 and PD1 are coupled together and to thedrains of transistors PU2 and PD2 to form a complementary first storagenode SNB1. Sources of the pull-up transistors PU1 and PU2 are coupled topower voltage CVdd, and the sources of the pull-down transistors PD1 andPD2 are coupled to a ground voltage CVss.

The first storage node SN1 of the first data latch is coupled to bitline BL through pass-gate transistor PG1, and the complementary firststorage node SNB1 is coupled to complementary bit line BLB throughpass-gate transistor PG2. The first storage node N1 and thecomplementary first storage node SNB1 are complementary nodes that areoften at opposite logic levels (logic high or logic low). Gates ofpass-gate transistors PG1 and PG2 are coupled to a word line WL.

As shown in the top view layout of FIG. 7B, the single-port SRAM cell500 includes a plurality of fin lines 510-513 (also referred to asactive region, or OD). The N-type fin lines 510 and 513 are comprised ofa non-germanium-containing semiconductor material, for example silicon.The P-type fine lines 511-512 are comprised of silicon germanium forstrain effect enhancement.

Similar to the SRAM cells discussed above with reference to FIG. 5, thefin lines 510 and 513 located over a P-type well region of the SRAM cell500 extend continuously in the X-direction, whereas the fin lines 511and 512 located over an N-type well region of the SRAM cell 500 extenddiscontinuously in the X-direction. In other words, the fin line 511 andthe fin line 512 each extend partially into, but not completely through,the SRAM cell 500. According to the embodiment shown in FIG. 7B, the finline 511 extends into the SRAM cell 500 from the “bottom” of the SRAMcell 500, and it terminates into the SRAM cell 500 on the drain side ofthe pull-up transistor PU1. The fin line 512 extends into the SRAM cell500 from the “top” of the SRAM cell 500, and it terminates into the SRAMcell 500 on the drain side of the pull-up transistor PU2. This type ofconfiguration helps prevent the data node leakage between the drainnodes of adjacent pull-up transistors.

FIG. 8A illustrates a cross-sectional side view of two abutting SRAMcells 500A-500B, and FIG. 8B illustrates the corresponding layout of thetwo abutting SRAM cells 500A-500B in a top view according to embodimentsof the present disclosure. The SRAM cells 500A and 500B are eachconfigured the same as the SRAM cell 500 in FIGS. 7A-7B. In FIG. 8B, theSRAM cell 500A is “flipped upside down” and joined with the SRAM cell500B, which is not flipped. In other words, the SRAM cells 500A and 500Bare symmetrically disposed around an axis 520.

As discussed above with reference to FIG. 7B, the NMOSFET fin lines 510and 513 (located over the P-type well region) extend continuouslythrough at least the two SRAM cells 500A-500B. In comparison, the SRAMcells 500A-500B have discontinuous PMOSFET fin lines. For example, thefin lines 511A-511B and 512 are PMOSFET fin lines located over theN-type well region and have the SiGe content. The fin line 511A extendspartially into the SRAM cell 500A but does not extend into the SRAM cell500B, the fin line 512 extends partially (but not completely) into boththe SRAM cells 500A and 500B, and the fin line 511B extends partiallyinto the SRAM cell 500B but does not extend into the SRAM cell 500A. Thefin lines 511A, 512, and 511B are also not connected to one another. Thediscontinuous fine lines 511A-511B and 512 each end in the drain side ofthe pull up transistors PU1 or PU2. As discussed above with reference toFIG. 7B, this type of broken fin line layout is used herein to preventor reduce data node leakage between the pull up transistor drain node ofone SRAM cell 500A and the pull up transistor drain node of an adjacentSRAM cell 500B.

The cross-sectional side view shown in FIG. 8A is obtained by cuttingthe top view of FIG. 8B along a cutline 530. Due to the location of thecutline 530, the fin line 512 is shown in the cross-sectional view ofFIG. 8A. The fin line 512 is located over an N well, which is formedin/over a substrate. Source and drain regions are formed in the fin line512, and gates for the pull up transistors PU1 and PU2 are formed overthe fin line 512. Contacts (CO) are formed over the source and drainregions to provide electrical connectivity thereto. The discontinuousnature of the PMOSFET fin lines is manifested in FIG. 8A as the fin line512 not extending fully laterally, for example not extending fully belowthe gates 550 and 551. Also as shown in FIG. 8A, both ends of the finline 512 terminate on their respective sides of the drains.

Another aspect of the present disclosure involves multiple work-functionmetals for the standard cells and the SRAM cells. This is illustrated inmore detail in FIGS. 9A and 9B, where FIG. 9A is a diagrammaticfragmentary cross-sectional side view of a portion of a CMOSFET device700 in a standard cell (e.g., as a part of the standard cells array 100discussed above), and FIG. 9B is a diagrammatic fragmentarycross-sectional side view of a portion of a CMOSFET device 701 in anSRAM cell (e.g., as a part of the SRAM cells array 200 discussed above).It is understood that the cross-sectional side views of FIGS. 9A and 9Bare obtained by cutting along the Y-direction in FIG. 1A. The PMOS andNMOS sections of the CMOSFET devices 700-701 are labeled in FIGS. 9A and9B.

The CMOSFET devices 700-701 each include a dielectric isolationstructure 710, for example shallow trench isolation (STI). The STD cellCMOSFET device 700 includes fin structures 720 and 721 that protrudevertically (e.g., in the Z-direction of FIG. 1A) out of the dielectricisolation structure 710. The fin structure 720 is a part of the PMOS ofthe STD cell CMOSFET device 700, and the fin structure 721 is a part ofthe NMOS of the STD cell CMOSFET device 700. The SRAM cell CMOSFETdevice 701 includes fin structures 730 and 731 that protrude vertically(e.g., in the Z-direction of FIG. 1A) out of the dielectric isolationstructure 710. The fin structure 730 is a part of the PMOS of the SRAMcell CMOSFET device 701, and the fin structure 731 is a part of the NMOSof the SRAM cell CMOSFET device 701. As discussed above, the finstructures 720 and 730 for the PMOS comprise silicon germanium (SiGe),whereas the fin structures 721 and 731 for the NMOS comprise anon-germanium-containing semiconductor material, such as silicon (Si).The channel regions of the CMOSFET devices 700 and 701 are formed in thefin structures 720-721 and 730-731.

The CMOSFET device 700 includes a gate dielectric layer 740 that isformed over the dielectric isolation structure 710 and over the finstructures 720-721, and the CMOSFET device 701 includes a gatedielectric layer 750 that is formed over the dielectric isolationstructure 710 and over the fin structures 730-731. In some embodiments,the gate dielectric layers 740 and 750 includes silicon oxynitride,silicon nitride, or silicon oxide. In other embodiments, the gatedielectric layer 740 and 750 includes a high-k dielectric material,which is a material having a dielectric constant that is greater than adielectric constant of SiO2. In an embodiment, the high-k gatedielectric material includes hafnium oxide (HfO2), which has adielectric constant that is in a range from approximately 18 toapproximately 40. In alternative embodiments, the high-k gate dielectricmaterial may include ZrO2, Y₂O₃, La2O5, Gd2O5, TiO2, Ta2O5, HfErO,HfLaO, HfYO, HfGdO, HfAlO, HfZrO, HfTiO, HfTaO, or SrTiO.

A P-type work function metal layer 760 is formed over the gatedielectric layer 740 in the PMOS region of the CMOSFET device 700, andan N-type work function metal layer 761 is formed over the gatedielectric layer 740 in the NMOS region of the CMOSFET device 700.Meanwhile, a P-type work function metal layer 770 is formed over thegate dielectric layer 750 in the PMOS region of the CMOSFET device 701,and an N-type work function metal layer 771 is formed over the gatedielectric layer 750 in the NMOS region of the CMOSFET device 701.

In some embodiments, the P-type work function metal layers 760 and 770each comprise a metal material that is titanium nitride (TiN) ortantalum nitride (TaN). It is understood that additional metal layersmay be stacked upon the P-type work function metal layers 760 and 770.In some embodiments, the N-type work function metal layers 761 and 771each comprise a metal material that is titanium nitride (TiN), titaniumaluminum (TiAl), titanium aluminum nitride (TiAlN), tantalum nitride(TaN), tantalum aluminum (TaAl), tantalum aluminum nitride (TaAlN),tantalum aluminum carbide (TaAlC), or tantalum carbon nitride (TaCN).

Also as shown in FIGS. 9A and 9B, the P-type work function metal layer760 has a thickness 780, and the P-type work function metal layer 770has a thickness 790. In some embodiments, the thickness 780 is greaterthan the thickness 790. In some embodiments, the thickness 790 is in arange between about 5 angstroms and about 80 angstroms, and thethickness 780 is in a range between about 5 angstroms and about 30angstroms. This thickness difference results in the P-type work functionmetal layer 760 having a lower threshold voltage Vt than the P-type workfunction metal layer 770. In some embodiments, the threshold voltage Vtassociated with the P-type work function metal layer 760 is 50 mV to 200mV smaller than the threshold voltage Vt associated with the P-type workfunction metal layer 770.

In some embodiments, the N-type work function metal layer 761 has alower threshold voltage Vt than the N-type work function metal layer771. This lower threshold voltage Vt is achieved by configuring thealuminum content of the work function layers 761 and 771. For example,the work function metal layer 761 may have a higher aluminum content(e.g., in the TaAl or TiAl compound) than the work function metal layer771. In some embodiments, the aluminum concentration for both the layer761 and 771 is in a range between about 2% and about 50%, though it isunderstood that the aluminum concentration is still higher for the layer761 than the layer 771. In some embodiments, by configuring the aluminumcontent differently for the work function metal layers 761 and 771, thethreshold voltage Vt associated with the N-type work function metallayer 761 is 50 mV to 200 mV smaller than the threshold voltage Vtassociated with the N-type work function metal layer 771. As such, theSRAM CMOSFET has higher threshold voltage Vt than the logic circuitCMOSFET (for both the PMOS and the NMOS). This is desired because theSRAM cells typically require a higher threshold voltage Vt than thestandard logic circuit cells.

A fill metal 800 is also formed over the work function metals 760-761and 770-771. The fill metal 800 serves as the main conductive portion ofthe gate electrode. In some embodiments, the fill metal 800 comprisestungsten (W). In other embodiments, the fill metal 800 comprisesaluminum (Al). The work function metal layers 760-761 and 770-771 andthe fill metal 800 collectively constitute the metal gate electrode forthe CMOSFET. A dielectric layer 810 also surrounds the metal gateelectrode. In some embodiments, the dielectric layer 810 comprises alow-k dielectric material.

FIG. 10 is a diagrammatic fragmentary cross-sectional side view of aportion of an interconnect structure 850 according to embodiments of thepresent disclosure. The interconnect structure 850 may be used tointerconnect the elements of the standard cells or the SRAM cellsdiscussed above. As illustrated in FIG. 10, the interconnect structure850 includes a plurality of metal layers, for example metal layers M1,M2, M3, and M4. Isolation structures such as shallow trench isolation(STI) are formed in the substrate. A plurality of gates is formed overthe substrate. Conductive contacts (CO) are formed over the substrateand over the gates. Some of these contacts are butted contacts (BTC). Aplurality of vias (such as via0, via1, via2, via3) provide electricalconnectivity between the metal layers and the gates (and othercomponents such as source/drain).

FIG. 11 is a flowchart illustrating a method 900 according to anembodiment of the present disclosure. The method 900 includes a step910, in which one or more continuous first fin lines are formed in alogic circuit cells array that includes a plurality of logic circuitcells abutted to one another in a first direction. The one or morecontinuous first fin lines are formed such that they each extend acrossat least three of the abutted logic circuit cells in the firstdirection.

The method 900 includes a step 920, in which discontinuous second finlines are formed in a static random access memory (SRAM) cells arraythat includes a plurality of SRAM cells abutted to one another in thefirst direction. The discontinuous second fin lines each extends into nomore than two of the abutted SRAM cells.

In some embodiments, each of the discontinuous second fin lines extendsacross no more than two of the abutted SRAM cells in the firstdirection.

In some embodiments, the discontinuous second fin lines include atleast: a first segment that extends partially into a first SRAM cell anda second SRAM cell abutted to the first SRAM cell; a second segment thatextends partially into the second SRAM cell and a third SRAM cellabutted to the second SRAM cell; and a third segment that extendspartially into the third SRAM cell and a fourth SRAM cell abutted to thethird SRAM cell. In some embodiments, the first segment is separatedfrom the third segment in the first direction by a first gap, the secondsegment is separated from the first segment or the third segment in asecond direction by a second gap, the second direction being differentfrom the first direction, and the first gap extends across a boundarybetween the second SRAM cell and the third SRAM cell.

In some embodiments, the SRAM cells array includes PMOSFETs andNMOSFETs; and the discontinuous second fin lines are fin lines for thePMOSFETs. In some embodiments, the SRAM cells array further includes oneor more continuous third fin lines for the NMOSFETs. In someembodiments, each of the continuous third fin lines extends across atleast three of the abutted SRAM cells in the first direction. In someembodiments, the discontinuous second fin lines each comprise silicongermanium; and the continuous third fin lines each comprise anon-germanium-containing semiconductor material.

In some embodiments, each of the SRAM cells includes a pull-uptransistor; and each of the discontinuous second fin lines terminates ina drain of the pull-up transistor.

In some embodiments, the logic circuit cells array further includes oneor more isolation transistors each located between two respectiveabutted logic circuit cells; and each of the isolation transistors isconfigured to provide electrical isolation between the two respectiveabutted circuit cells. In some embodiments, the isolation transistorsinclude a PMOSFET isolation transistor and an NMOSFET isolationtransistor; a gate of the PMOSFET isolation transistor is electricallytied to a Vdd voltage source; and a gate of the NMOSFET isolationtransistor is electrically tied to a Vss ground. In some embodiments,each of the isolation transistors includes a respective gate that islocated at a respective border between two abutted logic circuit cells.

In some embodiments, the logic circuit cells array and the SRAM cellsarray each include an NMOSFET and a PMOSFET; a gate of the PMOSFET ofthe logic circuit cells array includes a first work function metal; agate of the PMOSFET of the SRAM cells array includes a second workfunction metal; a gate of the NMOSFET of the logic circuit cells arrayincludes a third work function metal; a gate of the NMOSFET of the SRAMcells array includes a fourth work function metal; and at least one ofthe first, second, third, and fourth work function metals is differentfrom a rest of the first, second, third, and fourth work functionmetals. In some embodiments, the first work function metal is thickerthan the second work function metal. In some embodiments, the third workfunction metal has a greater aluminum content than the fourth workfunction metal.

It is understood that additional processes may be performed before,during, or after the steps 910-920 of the method 900. For reasons ofsimplicity, these additional steps are not discussed herein in detail.

Based on the above discussions, it can be seen that the presentdisclosure offers advantages over conventional FinFET SRAM devices. Itis understood, however, that other embodiments may offer additionaladvantages, and not all advantages are necessarily disclosed herein, andthat no particular advantage is required for all embodiments. Oneadvantage is that the discontinuous PMOS fin lines for the SRAM reducesthe strain effect, which suppresses the I_(on) current. The reduction ofthe I_(on) current improves SRAM write margins. Meanwhile, the fin linesfor the logic circuit cells are continuous. The continuous fin linesentail faster chip speed. Another advantage is that the isolationtransistors are implemented to provide electrical isolation betweenadjacent cells. Yet another advantage is that multiple work functionmetals are implemented for the logic circuit cells and the SRAM cells.The content and/or thicknesses of the work function metals areconfigured such that the SRAM MOSFETs have a greater threshold voltageVt than the logic circuit MOSFETs, which is also desired. Otheradvantages include compatibility with existing fabrication process flowand ease of implementation.

One aspect of the present disclosure pertains to an IC chip. The IC chipincludes a logic circuit cells array and a static random access memory(SRAM) cells array. The logic circuit cells array includes a pluralityof logic circuit cells abutted to one another in a first direction. Thelogic circuit cells array includes one or more continuous first finlines that each extends across at least three of the abutted logiccircuit cells in the first direction. The static random access memory(SRAM) cells array includes a plurality of SRAM cells abutted to oneanother in the first direction. The SRAM cells array includesdiscontinuous second fin lines.

Another aspect of the present disclosure pertains to a semiconductordevice. A plurality of logic circuit cells are adjacently disposed toone another in a first direction. A first fin line extends continuouslyacross at least three of the logic circuit cells in the first direction.A plurality of static random access memory (SRAM) cells are adjacentlydisposed to one another in the first direction. A plurality of secondfin lines each extend into no more than two of the SRAM cells. Thesecond fin lines are disjointed with one another. The second fin linesare PMOS fin lines. The second fin lines each comprise silicongermanium.

Yet another aspect of the present disclosure pertains to a method. Oneor more continuous first fin lines are formed in a logic circuit cellsarray that includes a plurality of logic circuit cells abutted to oneanother in a first direction. The one or more continuous first fin linesare formed such that they each extend across at least three of theabutted logic circuit cells in the first direction. Discontinuous secondfin lines are formed in a static random access memory (SRAM) cells arraythat includes a plurality of SRAM cells abutted to one another in thefirst direction. The discontinuous second fin lines each extend into nomore than two of the abutted SRAM cells.

The foregoing has outlined features of several embodiments so that thoseskilled in the art may better understand the detailed description thatfollows. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions andalterations herein without departing from the spirit and scope of thepresent disclosure. For example, by implementing different thicknessesfor the bit line conductor and word line conductor, one can achievedifferent resistances for the conductors. However, other techniques tovary the resistances of the metal conductors may also be utilized aswell.

What is claimed is:
 1. A device, comprising: a static random accessmemory (SRAM) cells array that includes: a first type well, a secondtype well disposed adjacent to the first type well in a first direction,a plurality of elongated first fin structures that each extend in asecond direction, wherein the first fin structures are disposed over thefirst type well; and a non-SRAM cells array that includes a plurality ofelongated second fin structures that each extend in the seconddirection, wherein the second fin structures each have a substantiallylonger dimension in the second direction than each of the first finstructures.
 2. The device of claim 1, wherein the SRAM cells arrayfurther includes a plurality of elongated third fin structures that aredisposed over the second type well.
 3. The device of claim 2, whereinthe third fin structures each have a substantially longer dimension inthe second direction than each of the first fin structures.
 4. Thedevice of claim 3, wherein: the first fin structures each extend over nomore than two SRAM cells; and the third fin structures each extend overat least three SRAM cells.
 5. The device of claim 1, wherein: the firsttype well includes an n-type well; the second type well includes ap-type well; and the first fin structures are each disposed over then-type well but not over the p-type well.
 6. The device of claim 5,wherein the SRAM cells array is free of having any fin structures thatare disposed over the p-type well.
 7. The device of claim 1, wherein thenon-SRAM cells array include a logic cells array.
 8. The device of claim1, wherein: the non-SRAM cells array includes a third type well and afourth type well; a first subset of the second fin structures aredisposed over the third type well; and a second subset of the second finstructures are disposed over the fourth type well.
 9. The device ofclaim 1, wherein: the non-SRAM cells array includes a plurality ofnon-SRAM cells that are arranged into a column; the non-SRAM cells arrayincludes a plurality of isolation transistors; and gate structures ofthe isolation transistors are located at borders between the non-SRAMcells.
 10. The device of claim 1, wherein: the SRAM cells array includesa plurality of SRAM cells that are arranged into a column; the first finstructures are separated from one another in the second direction by aplurality of gaps; and the gaps are disposed over borders between theSRAM cells.
 11. A device, comprising: a plurality of static randomaccess memory (SRAM) cells that are arranged into a first column thatextends in a first direction in a top view; and a plurality of non-SRAMcells that are arranged into a second column that extends in the firstdirection in the top view; wherein: the SRAM cells include a pluralityof first fin structures that each extends partially through two of theSRAM cells in the top view; and the non-SRAM cells include a pluralityof second fin structures that each extends through three or more of thenon-SRAM cells in the top view.
 12. The device of claim 11, wherein thesecond fin structures are each substantially longer in the firstdirection than each of the first fin structures.
 13. The device of claim11, wherein: the SRAM cells further include a plurality of third finstructures that each extends through three or more of the SRAM cells inthe top view; and the third fin structures are spaced apart from thefirst or second fin structures in a second direction perpendicular tothe first direction.
 14. The device of claim 13, wherein: the SRAM cellsinclude an n-type well and a p-type well; the first fin structures areeach disposed over the n-type well; and the third fin structures areeach disposed over the p-type well.
 15. The device of claim 11, wherein:the SRAM cells include an n-type well and a p-type well; the first finstructures are each disposed over the n-type well; and no fin structuresare disposed over the p-type well.
 16. The device of claim 11, whereinthe non-SRAM cells include a plurality of logic circuits and a pluralityof isolation transistors that provide electrical isolation betweenadjacent ones of the non-SRAM cells.
 17. The device of claim 11,wherein: the non-SRAM cells include an n-type well and a p-type well; afirst group of the second fin structures are formed over the n-typewell; and a second group of the second fin structures are formed overthe p-type well.
 18. The device of claim 11, wherein: the non-SRAM cellsinclude an n-type well and a p-type well; and none of the second finstructures is formed over the p-type well.
 19. A device, comprising: astatic random access memory (SRAM) that includes pull-up (PU)transistors, pass-gate (PG) transistors, and pull-down (PD) transistors,wherein: the PU transistors include a plurality of first fin structures,the PG transistors or the PD transistors includes a plurality of secondfin structures; and a plurality of non-SRAM that include a plurality ofthird fin structures; wherein: the PU transistors include a plurality offirst fin structures; the PG transistors or the PD transistors includesa plurality of second fin structures that are substantially longer thanthe first fin structures; and the third fin structures are substantiallylonger than the first fin structures.
 20. The device of claim 19,wherein: the non-SRAM includes a plurality of non-SRAM cells and aplurality of isolation transistors; and gate structures of the isolationtransistors are located at boundaries between adjacent ones of thenon-SRAM cells.